Coating composition for dry cells



Jam 1933- J. G. ZIIMMERMAN ,892,691

COATING COMPOSITION FOR DRY CELLS Filed April 19. 1929 INVENTOR PatentedJan. 3, 1933' UNITED STATES PATENT OFFICE JAMES GARFIELD ZIMMERMAN, OFMADISON, WISCONSIN, ASSIGNOR TO BURGESS BATTERY COMPANY, OF MADISON,WISCONSIN, A CORPORATION OF WISCONSIN COATING COMPOSITION FOR DRY CELLSApplication filed April 19,

This invention relates to a coated, dry cell, depolarizing core and moreparticularly to a dip or solution'capable of depositing upon such a corea closely adherent protective and electrically conductive coating.

The ordinary bag type dry cell, as illustrated in the single figure ofthe accompanying drawing, comprises a zinc shell 1, and a core 2. Thecore consists of a depolarizing mixture of manganese dioxide orotherdepolarizer, powdered graphite, sal ammoniac, zinc chloride, andwater. A carbon rod 3, having a brass cap 4, is embedded in thedepolarizing mixture. The core 2, with its embedded carbon rod 3, may betermed a cathode core or simply a core. The core is set in a gelatinouselectrolyte 5 which contacts with the zinc shell 1. The gelatinouselectrolyte may be that described in United States Patent No. 1,292,764.Itusually consists of sal ammoniac, zinc chloride, a cereal such asstarch, and water. A seal 6 of sealing wax or pitch, rests on washer 7and closes the top of the cell. An expansion space 8 may be left betweenwasher 7 and the top of the electrolyte 5.

In the production of core 2, a quantity of moist, loose depolarizingmixture iscompressed about the carbon rod in a mold. Usually there is nofurther cohesive force than that resulting from the moisture andcompression. As a result, during factory manipulation, particles of themixture frequently become dislodged at the surface of the core. Theseparticles may bridge the intervening annular space occupied byelectrolyte 5, make contact with zinc shell 1, and cause short circuitand local action within the cell. Prior to gelatinization of theelectrolyte, the core 2 absorbs moisture therefrom and gradually becomessoggy. The sogginess of the core may increase to a point where, ifgelatinization is greatly delayed, the core disintegrates and the cellis rendered useless.

To avoid such disintegration and short circuits, it is common practiceto envelop the core 2 in a closely adherent bibulous wrapper, usually ofcheesecloth. The cost of such a wrapper, together with the labor ofapplying it, represents a considerable item inthe 1929. Serial No.356,319.

cost of the dry cell. The gauze wrapper is, in itself, expensive. Itmust be cut into rectangular sections of a proper size to encircle acore twice. It must be treated with an adhesive and must be wrappedcarefully about the core to secure it in the correct position. Althoughgauze may be replaced by cheap tissue paper, the results are not entire-1y satisfactory. Cells have been made without any wrapper but theomission of the wrapper requires careful technique and even then thepercentage of rejects is high and the quality of the cells questionable.

It is an object of this invention to provide a retaining covering or dipfor dry cell cores which is less expensive than a bibulous cloth wrapperand is more easily applied.

It is a further object of this invention to provide an improved coveringfor dry cell cores which imparts improved characteristics, such as lessinternal resistance and longer life to a dry cell.

Many attempts have been made to provide a dip or covering formed bydipping the depolarizing core into a liquid solution or suspension.Examples of such dips are described in United States Patents Nos.1,316,597 and 1,370,052. Such dips comprise a non-conductive material,such as plaster of Paris, with or without an adhesive such as glue,starch and the like. To be of any service the coating of the core mustbe so thick as to result in excessive electrical resistance. If thecoating is too thin it does not provide the desired protection for thecore, especially after coming into contact with the liquid electrolyteprior to gelatinization.

My improved retaining covering or coating consists of a liquid medium ordip into which the core is dipped and which, upon removal of the core,adheres thereto to form a conductive layer 9 of uniform thicknessthereon. The coating, after being allowed to dry, becomes suflicientlytough, cohesive and water-resistant to prevent sogginess and thedislodgment of particles of depolarizEng mixture from the body of thecore. It, also, allows the necessary handling and stacking which thecores receive in the regular course of factory operations. It retainsits conductivity so that the internal resistance of the cells is notincreased.

My liquid coating medium comprises casein dissolved in an aqueoussolution of a casein-dissolving reagent. Casein is readily soluble indilute aqueous solutions of the alkali and alkaline earth hydroxides,and aqueous solutions of their soluble carbonates and bicarbonates. Itis soluble in dilute solutions of mineral and organic acids. Any of thementioned reagents, if kept weak enough not to impair the action of thecell, can be used for my purpose. Concentrated solutions are unsuitablesince they hydrolize and precipitate the casein and introduce ex"cessive corrosion of the zinc electrode.

Examples of casein-dissolving reagents include mineral acids, such ashydrochloric and sulphuric acids, organic acids, including those thatare relatively strong, such as formic and acetic acids, and those thatare comparatively weak, such as citric, lactic and malic acids. Mypreferred casein-dissolving reagent is citric acid which dissolvescasein qu te readily and does not exert a deleterious action upon thedry cell. The precise action of citric aci d upon casein is not known.It is believed, however, that a compound of the two is formed whichcompound is soluble in water but is believed to be colloidal.

I prefer to use only as much citric acid as is necessary to effectsubstantially complete solution of the casein. I may vary the proportionof casein in the solution from three to eight percent, but more thanthis maximum proportion results in a solution which is too thick forsatisfactory dipping at ordinary temperatures. I prefer to use a live tosix percent solution of casein. It is desirable to limit the proportionof citric acid in order to prevent unnecessary corrosion of the zincelectrode and I find that citric acid in amounts substantially equal tothe amounts by weight of casei is satisfactory. Thus, I may vary theproportion of citric acid from three to eight percent, while mypreferred proportion is from five to six percent, based upon thecrystalline form of the acid.

In making my improved coating solution or dip I proceed to mix thedesired proportions of powdered casein and water, preferably by startingwith the casein, adding cold water, and slowly agitating the mixture tomaintain a smooth consistency. T he mixture may be heated and thedesired quantity of citric acid added while heating. The temperatureshould not be allowed to exceed C., or objectionable chemical changeswill take place in the casein. After the casein has become substantiallycompletely dissolved, which should not require more than thirty minutes,the solution is ready for the dipping operation.

Casein solutions are viscous, and they be come more viscous as theproportion of casein increases while they become more limpid as thetemperature increases. I have found that a five to six percent solutionwill deposit a very satisfactory coating at ordinary room temperature.The core should be immersed for a period of about live seconds in orderthat a uniform coating be secured. A momentary dip results in the lowerportion being in the solution a period of time comparatively longer thanthe upper portion with the result that a thicker coating forms upon thelower portion. This effect is increased by the natural draining of thesolution toward the bottom before the coating sets. A drip bead usuallyforms upon the bottom of the core and this may be removed by contactwith any surface, preferably one which is slightly roughened. Momentarywiping contact with a brush removes the bead very efi'ectively. Thedipped core is then dried in the ordinary atmosphere. The usualexpedients may be used to accelerate drying. The temperature may not bematerially increased during the early stages of drying while the coatingis still wet since the coating will become thinner and will run. Thetemperature may not exceed 70 C. at any time or permanent injury to thecasein will result. Moisture migrates from the coating into the core tocontribute to the drying action. After the coating is dried the core maybe dipped into a fused wax dip to insulate the bottom of the core andform a spacing collar 10 thereon. The core may be dipped into theinsulating wax before the coating is applied. This wax collar 10prevents contact between the zinc can and the core. Instead of formingsuch an insulating bottom upon the core, an insulating washer may beused in the bottom of the can.

The salts in the depolarizing core, namely sal ammoniac and zincchloride, precipitate the casein and assist in forming a hard, leathery,porous wrap. The precipitated casein retards the movement of moisturefrom the coating to the core and prevents disintegration of the mixbefore the coating has set. The pores are very small which renders itimpossible for loose framents of the depolarizing mix to go through thecoating and contact with the zinc can to cause short circuits, which ispossible with a gauze wrapper. The coating, while possessing amplestrength to withstand the necessary factory manipulation, is stillthinner than the bibulous gauze wrapper and provides increased space forelectrolyte. The citric acid in the coating provides electricalconductivity and decreases the internal resistance of the cell. Mycoating slowly disintegrates in water but disintegration is delayed fora number of minutes. In contact with a fresh liquid electrolyte such asis referred to heretofore, distintegration takes place more slowly and,since the electrolyte sets into a firm gelatinous condition within a fewminutes, the coating preserves the core in compact form until after theelectrolyte has set. After this the electrolyte supports the core andprevents deformation'thereof.

It has also been proposed to provide a coat ing composition for dry cellcores comprising a liquid suspension of starch in a water solution ofgelatinized starch, a specific composition being a suspension of 30% to55% of starch in solution of 0.5% to 1.5% of gelatinized starch. I havediscovered that my coating composition may be very advantageouslymodified by admixing therewith varying proportions of the starch coatingcomposition. I may add from 5% to 15% ammonium chloride to thecomposition to decrease the viscosity of the solution. The resultingcomposition is given a very smooth consistency and, upon dipping, itdistributes itself quite uniformly over the surface of the core.

While I have described my improved dip in its relation to a cylindricaldry cell core, it may be applied to the cores of other forms of drycells, as for instance, those used in the plate type of battery. Themethod of application also may be varied. It may be applied to thedifferent types of cathodes by painting, spraying, or by any othersuitable method.

I claim:

1. A composition for coating dry cell cores comprising a liquidsuspension of starch in a Water solution of gelatinized starch, caseinand a casein-dissolving reagent.

2. A composition for coating dry cell cores comprising a liquidsuspension of starch in a Water solution of gelatinized starch, caseinand a weak organic acid.

3. A. composition for coating dry cell cores comprising a liquidsuspension of starch in a water solution comprising gelatinized starch,ammonium chloride, casein and citric acid.

In testimony whereof I aiiix my signature.

JAMES GARFIELD ZIMMERMAN.

